As electronic products continue to become smaller and thinner, more electronic components with higher integration and smaller pin spacing are being applied to PCBA motherboards. This trend has led to increasingly high requirements for surface mount technology (SMT) assembly processes. In order to ensure the welding quality of products, it is necessary to strictly follow the technical standards of SMT assembly.
① SMT Surface Mount Technology Quality Inspection Standards
⑴ The Definition of Standard.
【Accept Criterion】: The Accept Criterion includes three conditions: Target Condition, Accept Condition, and Reject Condition.
【Target Condition】: The Target Condition represents the near-perfect assembly result, with high reliability in assembly. It is considered the ideal condition.
【Accept Condition】: The Accept Condition indicates the assembly result that has not reached the Target Condition, but can still maintain high assembly reliability. It is deemed as a qualified condition and falls under the Accept Criterion.
【Reject Condition】: The Reject Condition refers to the assembly result that does not meet the standard and may affect the product’s functionality. However, due to aesthetic factors to maintain the competitiveness of the company’s products, it is determined as a reject condition.
⑵ The Definition of Defect.
【Critical Defect】: A Critical Defect refers to a defect that is capable of causing harm to human beings or machines, or endangers the safety of life and property. It is denoted as CR.
【Major Defect】: A Major Defect refers to a defect that has caused the loss of practicality in the essential functions of the product, or reduced its reliability, leading to product damage or malfunction. It is denoted as MA.
【Minor Defect】: A Minor Defect refers to a defect that does not significantly affect the practicality of the product, and the product can still achieve the expected purpose. It is generally related to differences in appearance or mechanical assembly and is denoted as MI.
Quality inspection standards can provide guidance for the production process and quality assurance of products, which is of great significance for improving the quality of soldering.
② SMT Surface Mount Electrostatic Management Standard
IPC-ESD-2020: Joint standard for the development of electrostatic discharge control programs, including the necessary design, establishment, implementation, and maintenance of electrostatic discharge control programs. Provides guidance for handling and protecting during electrostatic discharge sensitive periods based on the historical experiences of some military and commercial organizations.
IPC-SA-61A: Handbook for post-soldering aqueous cleaning. Covers various aspects of aqueous cleaning, including chemical and production residues, equipment, processes, process control, and considerations for environment and safety.
IPC-AC-62A: Handbook for post-soldering water-based cleaning. Describes the type and nature of manufacturing residues and water-based cleaning agents, the process, equipment, and processes of water-based cleaning, quality control, environmental control, and employee safety, as well as the determination of cleanliness and measurement costs.
IPC-DRM-40E: A desktop reference manual for through-hole solder joint evaluation. Provides detailed descriptions of component, hole wall, and solder pad coverage, as well as computer-generated 3D graphics, according to standard requirements. Covers solder filling, contact angle, wetting, vertical filling, pad coverage, and numerous solder joint defects.
IPC-TA-722: Handbook for soldering technology assessment. Includes various aspects of soldering technology, including general soldering, soldering materials, manual soldering, batch soldering, wave soldering, reflow soldering, vapor phase soldering, and infrared soldering.
IPC-7525: A design guide for stencils. Provides guidance for the design and manufacture of stencils for solder paste and surface mount adhesive application. Also discusses stencil design for surface mount technology applications and introduces combined technologies with plated-through-hole or inverted chip components, including printing, double printing, and staged stencil design.
③ SMT Surface Mount Technology Cleaning Standards
When PCBs have issues such as bridging, insufficient, missing, excessive, or peaked solder after solder paste printing, or when they have been left on the production line for more than an hour, they need to be cleaned and reprinted.
PCB cleaning steps:
- Use a small scraper to remove the solder paste from the PCB and dispose of it in the waste solder paste box.
- Wet the lint-free wiping paper with a cleaning solution.
- Hold the PCB with the left hand and wipe the surface with the lint-free wiping paper using the right hand.
- After wiping, use an air gun to blow dry the PCB and remove any remaining solder paste in small holes, screw holes, etc.
- Defective PCB must be cleaned within one hour.
- In the cleaning area, separate the PCBs to be cleaned from the already cleaned ones.
- Ensure that the pinholes, screw holes, and other hard-to-reach areas are thoroughly cleaned.
There are many processes involved in SMT assembly, and even a small factor can easily cause product defects. Only by implementing strict SMT assembly technical standards can production be standardized and standardized, and product defects can be reduced.